The present invention relates to improved silver-glass paste compositions useful for attaching semiconductive elements, e.g., silicon dies, to appropriate substrates, and to a method for forming electronic components using such pastes.
Prior patents directed to similar pastes include U.S. Pat. Nos. 3,497,774; 4,401,767; 4,436,785; 4,459,166; 4,636,254 and U.S. Pat. No. 4,761,224. Typically, these pastes are used for attaching silicon dies to ceramic substrates.
Other improved silver-glass paste compositions are described in U.S. Ser. No. 07/248,120, filed Sept. 23, 1988 now U.S. Pat. No. 4,986,849, and U.S. Pat. No. 4,996,171, the subject matter of the application and patent being incorporated herein by reference.
The pastes known in the art typically include the following essential components in the approximate ranges indicated below:
______________________________________ Component % by weight ______________________________________ silver flake 55-80 glass frit 10-25 (e.g. lead borate glass) resin or resin-forming 0.0-2 component (e.g. a methacrylate) organic vehicle 5-20 ______________________________________
Other additives, e.g., silver oxide, thixotropic agents, or the like also may be included.
In a representative die-attachment process, the paste comprising silver flake, glass frit, resin and vehicle is placed in a cavity in a ceramic substrate, the die is placed on the paste and the resulting die/substrate package is fed on a belt onto and through a furnace where the package is heated to remove the organic vehicle and sinter the residual silver and glass to bond the die firmly to the substrate. The final bond layer must be completely free of voids and, as a consequence, the process usually requires a preliminary drying step in which vehicle is evaporated, followed by firing in a furnace to remove residual vehicle and melt the glass.
The preliminary drying step of necessity is quite lengthy, requiring between about 2-10 hours at 60.degree.-80.degree. C., depending on, for example, the size of the die and the surface area of the silver flake particles. Additionally, the ramp rate, i.e., the rate at which the package is fed from the drying step into the furnace, is carefully controlled so as to ensure that organic burnout is at least essentially completed before the sintering of the silver-glass mix takes place. Relatively low temperature (e.g., up to 50.degree. C./minute) ramp rates are very commonly used to insure vehicle removal and optimum results. Belt-type furnaces are normally employed for the firing stage and, depending on the number of temperature zones involved, the dwell time in the furnace can vary from 30-90 minutes or more.
When a large die is bonded, e.g., a die of 400-500 square mils or more, a large amount of bonding agent is needed, and, consequently, a significant amount of vehicle must be evaporated. To insure sufficient evaporation of the vehicle, the drying process may be particularly lengthy and the required ramp rate may be relatively low. The drying time can be decreased by using small silver flakes, which are packed more densely than large flakes and thus permit a faster rate of evaporation. However, when small flakes are used, cracks are likely to form in the bond because sintering tends to proceed too quickly. Thus, effective attachment of a large die to a substrate using conventional methods generally requires the use of large silver particles, and, therefore, a lengthy drying process and use of a relatively low temperature ramp rate.